The removal of nitrogen oxides (NOx) from internal combustion exhaust is an increasing concern, especially for lean-burn engines such as direct injection gasoline engines and Diesel engines. One method for post combustion NOx removal is the subjection of the exhaust gases to a non-thermal plasma process. In this regard, the exhaust gas is passed through a plasma processing tube whereat a high voltage electric field imparts formation of a plasma. The plasma has a large number of energetic electrons which collide with exhaust gas molecules to form atoms, ions and radicals. These atoms, ions and radicals, in turn, react either with the NO to make NO2, or with hydrocarbons to produce aldehydes. The produced aldehhydes subsequently reduce NO2 over suitable catalysts to make harmless nitrogen. Thus, the major role of the plasma reactor is to produce NO2 from NO and aldehydes from hydrocarbons in the combustion exhaust stream. Among the aldehydes produced in the plasma reactor, acetaldehyde (CH3CHO) is known to be the most effective for NO2 reduction over alkali-based catalysts.
FIGS. 1A through 1C depict three prior plasma reactors, wherein the external high voltage source is either pulsating D.C. or A.C.
FIG. 1A depicts a first form of plasma reactor 10, referred to commonly as a pulsed corona discharge plasma reactor, in which a conductive metallic tube 12 defines the reactor wall 14, and inside of which exhaust gas G passes along. Axially along the concentric center of the tube 12 is a conductive high voltage electrode rod 16. The central electrode rod 16 is electrified by an external high voltage source with the tube 12 serving as the ground electrode, wherein a corona is formed therebetween without sparking which induces plasma formation of the exhaust gas.
FIG. 1B depicts a second form of plasma reactor 10′, referred to commonly as a dielectric barrier discharge plasma reactor, in which a conductive metallic tube 22 and an insular dielectric layer 24, which is concentrically disposed at the inside surface of the tube collectively define the reactor wall 26. As in the first form of plasma reactor 10, exhaust gas G passes along the interior of the reactor wall 26, and a conductive high voltage electrode rod 28 is located axially along the concentric center of the tube 22. The central electrode rod 28 is electrified by an external high voltage source with the tube 22 serving as the ground electrode, wherein the dielectric layer 24 becomes polarized. The polarization of the dielectric layer 24 stores energy which serves to aid the inducement of the plasma formation of the exhaust gas without sparking.
FIG. 1C depicts a third form of plasma reactor 10″, commonly referred to as a dielectric packed-bed discharge plasma reactor, in which, as in the second form of plasma reactor 10′, a conductive metallic tube 32 and an insular dielectric layer 34, which is concentrically disposed at the inside surface of the tube, collectively define the reactor wall 36, wherein exhaust gas G passes along the interior of the reactor wall 36, and a conductive high voltage electrode rod 38 is located axially along the concentric center of the tube 32. A plurality of small insular dielectric pellets 40 loosely fill the interior of the reactor wall 36 such that the exhaust gas G is easily able to travel through the spaces therebetween. The central electrode rod 38 is electrified by an external high voltage source with the tube 32 serving as the ground electrode, wherein the dielectric layer 34 becomes polarized, and each of the pellets 40 becomes locally polarized, as well. The polarization of the dielectric layer 34 and of the local polarization of the pellets 40 store energy which serves to aid the inducement of the plasma formation of the exhaust gas without sparking.
In the prior art, the plasma reactor wall may have either a flat or cylindrical geometry, and the electrodes are typically made of continuous electrical conductors, so that a uniformly active electrical field is formed in the air gap therebetween to generate a plasma of maximum intensity for a given voltage. Prior art plasma reactors emphasize production of a high intensity plasma based on an implicit assumption that the plasma intensity is the limiting factor of the underlying process. The continuous electrodes utilized in the prior art plasma reactors may be suitable for operating conditions where the supply of high energy electrons is the rate limiting step of the plasma reaction. However, when the rate limiting step is other than the electron supply, an increase in input energy above a certain value through the continuous electrodes will hardly improve the overall performance of the plasma process.
The inventors of the present invention, while investigating plasma assisted lean NOx catalysis, have discovered that the limiting factor of the plasma reaction process is not the intensity of the plasma but the diffusion, mass transfer and chemical reaction of intermediates (such as atoms, ions and radicals) produced in the plasma under the operating conditions of a typical automotive engine exhaust gas stream. Thus, it is important to promote the diffusion, mass transfer and chemical reaction processes of atoms, ions and radicals in the plasma reactor in order to improve the overall performance of the NOx reduction process in the engine exhaust. In this regard, it is noted that energy is invested in the dielectric layer of prior art plasma reactors without an efficient pay-out with respect to the plasma energy in terms of encouraging maximal reaction of the atoms, ions and radicals with respect to the NOx and hydrocarbons.
Accordingly, what remains needed in the art of plasma reactors is to somehow provide an operative configuration which efficiently promotes diffusion, mass transfer and chemical reaction processes of atoms, ions and radicals in a plasma generator for automotive exhaust gas applications.